The base station in wireless communication systems for mobile phones and the like can be separated into an RE (radio equipment) that handles radio signals and an REC (radio equipment control) that controls the RE. One interface that connects the REC and the RE is the CPRI (Common Public Radio Interface) (refer to non-patent literature 1: “CPRI Specification V2.0”, online, obtained on Sep. 13, 2006 on the Internet at http//www.cpri.info/jp/spec.html).
With the separate RE and REC in the base station, the open interface CPRI allows components supplied from a variety of vendors to be used in the base station. The specified application range of the CPRI includes hardware layers L1 and L2.
FIG. 10 illustrates the CPRI of the base station. In FIG. 10, a base station 101 and a wireless terminal 111, which may be a mobile phone, for instance, are included.
The base station 101 can be separated into RE 102 and REC 103, as illustrated in FIG. 10. The RE 102 and the REC 103 are connected by the CPRI. By connecting the RE 102 and the REC 103 with the open CPRI, the base station 101 can include components supplied from a plurality of vendors.
FIG. 11 illustrates an outline of the CPRI protocol structure. The CPRI allows data exchange between the REC and the RE in accordance with the protocol structure as illustrated in FIG. 11. For example, “IQ Data” in FIG. 11 is user data of a digital base band signal. “SYNC” (synchronization) is a parameter for frame synchronization or time synchronization. “L1 Inband Protocol” is a link protocol used on a physical layer, and the protocol performs processing related to layer L1 at system startup or the like. “Control & Management Plane” is a protocol used for control management information transfer between the REC and the RE. “Vendor Specific” is a protocol used for vendor-specific information transfer, and this area can be used freely by the user.
FIG. 12 illustrates a first example of the CPRI frame structure. As illustrated in FIG. 12, the CPRI frame has a layered structure formed of basic frames, hyper frames, and a UMTS (Universal Mobile Telecommunications System) radio frame. A group of 256 basic frames forms a hyper frame, and a group of 150 hyper frames form the UMTS radio frame. The hyper frames are given indexes Z0 to Z149, and the basic frames are given indexes X0 to X255.
The basic frames form the basis of the CPRI frame structure, and the length of a single basic frame is defined as 1 Tc=1/3.84 MHz=260.416667 ns. A unit of the basic frame has sixteen 8-bit words, as illustrated in FIG. 12, and the basic frames are managed with values Y0 to Y3. The example illustrated in FIG. 12 uses Y0 (Y1 to Y3 will be described later).
The first byte of the basic frame is defined as a control word. The control word includes a hyper-frame index value, Z0 to Z149, a basic-frame index value, X0 to X255, and a basic-frame line number, Y0 to Y3.
The location of the basic frame in the UMTS radio frame can be indicated by a Z.X.Y (Z, X, and Y are positive integers) value specified in the control word. The Z.X.Y value can be understood as the address of the basic frame. In the subsequent description, the Z.X.Y value may be sometimes referred to as a control word address.
FIG. 13 illustrates the control word address. As illustrated in FIG. 13, the control word address indicating the location of each basic frame is placed in the control word (of the basic frame) in the hyper frame.
For example, suppose that the control word of a basic frame has ‘Z.1.0’ (Z is a given value). This indicates that the basic frame having the control word address ‘Z.1.0’ is placed in the hyper frame having the index ‘Z’ in the UMTS radio frame, and the basic frame has the index ‘X1’ and the line number ‘Y0’.
FIG. 14 illustrates a second example of the CPRI frame structure. The CPRI frame illustrated in FIG. 14 differs from the CPRI frame illustrated in FIG. 12 in that there are two basic-frame line numbers, ‘Y0’ and ‘Y1’. In the CPRI frame structure illustrated in FIG. 14, the control word address can have one of two Y values, ‘0’ and ‘1’.
FIG. 15 illustrates a third example of the CPRI frame structure. The CPRI frame illustrated in FIG. 15 differs from the CPRI frame illustrated in FIG. 12 in that there are four basic-frame line numbers, ‘Y0’ to ‘Y3’. In the CPRI frame structure illustrated in FIG. 15, the control word address can have one of four Y values, ‘0’ to ‘3’.
FIG. 16 illustrates a CPRI line bit rate. The CPRI line bit rate depends on the Y value of the line number. As indicated in FIG. 16, if a single Y value ‘Y0’ is possible, the line bit rate is 614.4 Mbps. If two Y values ‘Y0’ and ‘Y1’ are possible, the line bit rate is 1228.8 Mbps. If four Y values ‘Y0’ to ‘Y3’ are possible, the line bit rate is 2457.6 Mbps.
In the example of the CPRI frame structure illustrated in FIG. 12, the line bit rate is 614.4 Mbps. The example of the CPRI frame structure illustrated in FIG. 14 has double data in the basic frame, and the line bit rate becomes two times 614.4 Mbps, which is 1228.8 Mbps. The example of the CPRI frame structure illustrated in FIG. 15 has quadruple data in the basic frame, and the line bit rate becomes four times 614.4 Mbps, which is 2457.6 Mbps.
The 256 control words in each hyper frame form a single group, and the location of each control word (among the 256 control words) determines its use.
FIG. 17 illustrates control word mapping. CPRI protocols are allocated to each hyper frame as illustrated in FIG. 17.
For example, the basic frames having control word addresses ‘Z.0.0’, ‘Z.64.0’, ‘Z.128.0’, and ‘Z.192.0’ are assigned a Sync & Timing protocol. In the corresponding locations, predetermined information about the Sync & Timing protocol is placed. The basic frames having control word addresses ‘Z.2.0’, ‘Z.66.0’, ‘Z.130.0’, ‘Z.194.0’ are assigned the L1 inband protocol. In the corresponding locations, predetermined information about the L1 inband protocol is placed.
The control words in a single hyper frame form a single group. The CPRI protocol structure can be recognized only after the entire 256 control words are received.
“Sync & Timing” in FIG. 17 corresponds to “SYNC” and “Time Division Multiplexing” in FIG. 11. “L1 Inband Protocol” in FIG. 17 corresponds to “L1 Inband Protocol” in FIG. 11. “Vendor Specific” in FIG. 17 corresponds to “Vendor Specific” in FIG. 11. “Slow C&M” and “Fast C&M” in FIG. 17 correspond to “Control & Management Plane” in FIG. 11.
In the mapping in FIG. 17, the CPRI line bit rate is 614.4 Mbps, that is, there is one line number ‘Y0’. Even if the line bit rate is different, the control words of a single hyper frame form a group and are assigned the CPRI protocols.
Sectors and cells for the base station will be described next.
FIG. 18 illustrates the sectors and cells for the base station. As given in FIG. 18, the area in which the base station performs wireless communication with a wireless terminal is divided into sectors A to C. Each sector is divided into carriers f1 to f3.
A cell D is identified by a combination of the sector, A to C, and the carrier, f1 to f3. The example illustrated in FIG. 18 has nine cells D.
A single CPRI sector is assigned a single CPRI link. Therefore, the CPRI link will not be used to transfer the data of a different sector.
FIG. 19 illustrates the relationship between the sectors and the CPRI links. FIG. 19 illustrates the RE 102 and the REC 103 given in FIG. 10. As illustrated in FIG. 19, the RE 102 and the REC 103 are connected by n CPRI links 121a, 121b, 121c, . . . , and 121n. 
The CPRI transfers data with a single sector assigned to a single CPRI link. For instance, sector A in FIG. 18 is assigned to the CPRI link 121a in data transfer; sector B is assigned to the CPRI link 121b in data transfer; sector C is assigned the CPRI link 121c in data transfer.
In short, a single CPRI link can transfer the data of a plurality of cells. A data transfer rate of up to 2457.6 Mbps is possible per CPRI link. However, as described above, the CPRI link may not transfer the data of another sector. In the example given above, the data of sector A may not be transferred by the CPRI link 121b or 121c. 
In the CPRI, the CPRI link may not transfer the data of a different sector. If data is concentrated on a single CPRI link, the concentrated data may not be distributed to another CPRI link even if the link has a room.
Accordingly, the REC-RE data transfer capability depends on the maximum data transfer capability of the single CPRI link and may not meet the demands of greater transfer capabilities.